Calibration in measurement technology is the evaluation of measurement values that is delivered by a device that can deliver a calibration standard of known accuracy. Think of this standard as another measurement device of known accuracy. This device would generate the quantity that would be measured, such as voltage.
The outcome of the comparison can result in three possible outcomes: no significant error is noted on the device that is being tested, a significant error was noted but no adjustments were made, or an adjustment was made to correct the error to an acceptable level.
The International Bureau of Weights and Measures have established their own definition of calibration. BIPM defines calibration as “operation that, under specified conditions, in a first step, establishes a relation between the quantity values with measurement uncertainties provided by measurement standards and corresponding indications with associated measurement uncertainties (of the calibrated instrument or secondary stand) and, in a second step, uses this information to establish a relation for obtaining a measurement result from an indication.”
To break this definition down, it simply states that a calibration is simply a comparison, but it also introduces the concept of measurement uncertainty in relating the accuracies of the device being tested.
Across the world the increasing necessity for accurate results and the need for common international standards has prompted the establishment of national laboratories. Many countries have adopted the idea of building a National Metrology Institute (NMI) which helps to maintain the primary standards of measurement (the main SI units as well as many derived units). These facilities will also be used to provide traceability to a consumer’s measurement tools by calibration.
When an NMI is established in a country it establishes standards and regulations that span from top level measurements to simple measuring tools. As previously stated the NMI have established themselves all over the world. These include NIST in the United States, NPL in the UK, PTB in Germany and many more. Ever since the Mutual Recognition Agreement was signed it’s now easier than ever to take traceability from any other NMI and it’s no longer necessary for private companies to receive traceability from the NMI in the country where they’re located.
The quality of a calibration is always very important. To improve the quality of the calibration and to achieve the results that large organizations would find acceptable, it’s generally viewed as a desirable action for all measurements, including calibration, to be “traceable” to the units defined by the international community. To establish traceability, you must make a formal comparison to a standard that can be compared to national standards, certified reference materials, or international standards. This can be accomplished by private firms offering metrology services or national standards laboratories operated by the government.
When it comes to quality management systems, an effective metrology system is needed. This system requires that measuring tools need to be checked on a periodic basis. The standards ISO 9000 and ISO 17025 state that traceable actions are to a high level.
Often times communicating the quality of a calibration requires that the calibration value itself is often times supplemented by a uncertainty statement. Uncertainty analysis is what’s needed to evaluate this process. At times it may even require a DFS (Departure From Spec) to run machinery when it’s in a used state. This can only be authorized in writing by a manager along with a calibration technician.
Instruments and measuring devices fall into categories that focus on the actual quantities they are designed to measure. Altogether, these instruments and measuring devices span a wide range of categories such as time and frequency (intervalometer), electromagnetic radiation (RF probes), light (light meter), ionizing radiation (Geiger counter), mechanical quantities (pressure switch, pressure gauge, limit switch), and thermodynamic or thermal properties (temperature controller, thermometer). Each test device can vary in terms of the standard instrument used to attain a measurement such as a dry block temperature tester for a temperature gauge calibration and a dead weight tester for a pressure gauge calibration.
There are a number of prompts that would require a calibration. Here are a few:
When an instrument has been modified or fixed
Receiving a new instrument
After a specific period of time has passed (Cal certificate frequency of calibration required)
When certain observations may appear to be sketchy or when an instrument’s indications don’t match the output of surrogate instruments
Before and after an important measurement has been taken